Protectoseal Series No. 52500 Manhole Cover with pressure and vacuum relief, provides both emergency pressure relief and vacuum relief when installed on the manhole of a storage tank. In emergency situations (external fire involvement of the tank), normal venting devices may be too small to prevent overpressurization of the tank. The Series No. 52500 provides the necessary pressure relief capacity to maintain the tank in a safe operating range. The integral vacuum vent provides additional protection against tank collapse due to excessive vacuum in the vapor space..